And now news about something that surprised us. At first glance, it sounds great. You get paid thousand of dollars by doctors to eat your favorite fast food every single day. A bold new experiment. What do they hope to learn? John donvan with what they are already learned so far. Reporter: Hitting the fast food drive-in for science. Can I get five soft tacos. Can I get a number four? Reporter: Over and over and over again. It's been dave giocolo's life the past three months or so. As has been eating the food. A precisely measured 1,000 calories a day -- an extra meal daily -- fast food only. How many calories? 770. Reporter: Why? Because this man is paying him to. Dr. Samuel klein is a researcher at washington university medical school in st. Louis, trying to understand why weight gain leads to diabetes and hypertension. At some point, research on rats alone just isn't enough. Somebody has to eat this food this way to -- ultimately, it has to come to people. Reporter: And a radio ad -- attention overweight volunteers -- Reporter: -- That offered a cash incentive, up to $3,500. Once I got into work, i called them right away. Reporter: So did nurse dawn freeman. It probably took a month to get approved. Reporter: And so they ate. Every day -- from one of these five restaurants. Until their weight went up a target of 5% or 6%. Dawn, in eight weeks went from 170 to 186. Doesn't look like a lot. But here's how it felt. For two weeks it was all right, and then I started really feeling awful. I could hardly breathe any more. Reporter: Same thing for dave. Because he learned, weight is mysterious. It doesn't go up in a straight line at all. He actually lost one week. It took him three months to put on 18 pounds. And, near the end -- you just want this thing to be over. Yeah, I'm done with it now. I'm done with it. Reporter: This may help find therapy to obesity related dtsdss. Dave will have to take the weight off. That is a step he has to take to get the last $50. And you'll hear more of john's report on "nightline."

This transcript has been automatically generated and may not be 100% accurate.

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